Voltage regulation for electrostatic generators



March 4, 1952 +L w. BOUSMAN VOLTAGE REGULATION FOR ELECTROSTATICGENERATORS 2 SHEETS- SHEET l Filed June 27, 1950 Inventor: Henr'j'wBousman QH )QJ 4 His Attornefg.

March 4, 1952 H, w BQUSMAN 2,588,081

VOLTAGE REGULATION FOR ELECTROSTATIC GENERATORS Filed June 27 1950 2Sl-IEETS-SHEET :2

Inventor: Henrfg Bousman,

hrs 1 im His Attornefg.

Patented Mar. 4, 1952 VOLTAGE REGULATION FOR ELECTRO- STATIC GENERATORSHenry W. Bousman, Scotia, N. Y., asaignor to General Electric Company, acorporation of New York Application June 27, 1950, Serial No. 170,664

11 Claims. (Cl. 322-2) This invention relates generally to electrostaticgenerators and, more particularly, to systems and means for regulatingthe terminal voltage of electrostatic generators.

Electrostatic generators have long been utilized in certain applicationsfor the production of high voltages. Such apparatus usually comprises amoving dielectric belt upon which charges are sprayed by an ionizingelectrode. These charges are carried by the dielectric belt through adesired distance and then are collected within a rounded hollow terminalelectrode. As the belt continues to deposit chargesupon the terminalelectrode, the voltage thereof builds up until the charge deposited bythe belt is equaled by the amount of charge leakage from the terminalelectrode.

In recent years, the increased interest in the field of nuclear physicshas caused considerable attention to be drawn to the aforementioned typeof electrostatic generator as a means oi' supplying high voltages forthe acceleration of charged particles, e. g. protons or electrons. 'I'heemployment of an electrostatic generator for such Ia purpose isdescribed in an article entitled Control Equipment for 2.5 m. e. v. VanDe Graaf Giving an Ion Beam Constant to 1.5 k. e. v., appearing in thePhysical Review, volume 70, page 117 (1946), and written by J. L.McKibben, D. H. Frisch, and J. M. Hush. This article, along with othersdealing with similar subject matter, point out, however, that in orderfor the electrostatic generator. to be most useful in the eld of nuclearphysics, its terminal voltage must be closely regulated. 'Ihisrequirement is predicated upon the necessity of producing essentiallymono-energetic neutron beams from certain nuclear reactions resultingfrom the boinbardment of a given material with protons. If the terminalvoltage of the electrostatic generator varies, the energy of theaccelerated charged particles, or protons, will vary accordingly and, ifthe proton energy variation exceeds certain limits, a mono-energeticneutron beam can no longer be produced.

It is, therefore, an object of the present invention to provide systemsand means for maintaining the terminal voltage of an electrostaticgenerator within desired close limits under either free-running or loadconditions.

In accordance with one aspect of the invention more fully described anddisclosed hereinafter, capacitive means are provided in an electrostaticgenerator to be responsive to variations in the terminal electrodevoltage. In this manner, a signal substantially instantaneouslyresponsive to variations oi' the terminal electrode voltage is obtainedand fed back through'a feedback loop to a controllable high voltagesupply which provides voltages for a corona leak electrode. The voltageupon the corona leak electrode determines the amount of leakage currentdown from the terminal electrode and, hence, provides a means of varyingthe terminal electrode voltage. Consequently, the present'inventionprovides systems and means forl controlling the terminal voltage of anelectrostatic generator within close limits.

Other objects and advantages of this invention will be apparent from thefollowing description taken in connection with the accompanying drawingin which Fig. 1 is a schematic illustration of electrostatic generatorapparatus and control means therefor suitably embodying the invention;and Fig. 2 is a more detailed view of the capacitor plate shown in Fig.1.

Referring now to the drawing, there is shown electrostatic generatorapparatus comprising an airtight gas-filled enclosing tank I, atraveling charge carrier or belt 2 and a hollow terminal electrode 3.The flexible dielectric belt 2 is supported and driven in the directionindicated by the arrows by means of pulleys 4 and 5. Belt 2 may bedriven at an essentially constant speed by a motor 0 suitably attachedto pulley I. Positive charges are sprayed upon belt 2 by an ionizingelectrode 1 which may comprise a series of needles suitably supportedfrom tank I by means of an insulator 8. Positive potential with respectto grounded pulley 4 is applied to ionizing electrode 1 from a highvoltage supply 9 which may be controlled in a manner more fullydescribed hereinafter to determine the amount of charge sprayed uponbelt 2 by ionizing electrode 1.

After the charges have been sprayed upon belt 2, they are transported toa suitable position within collector electrode 3 where they arecollected by a collector electrode l0, which may comprise a series ofneedles, and transferred to terminal electrode 3 in a manner well knownto those skilled in the art. This process of charge accumulation uponterminal electrode 3 continues until the charge leakage away fromterminal electrode 3 and belt 2 to various portions of the apparatuscounterbalances the amount of charge sprayed upon belt by electrode 1.The resultant high voltage oi terminal electrode 3. which may be of theorder of 3,000,000 volts, may then be employed as a means ofaccelerating ions along ion accelerating tube I I. Ions are provided byan ion source I2 which may be evacuated through an exhaust tube I3attached to vacuum pump I4. Ion source I2 may comprise means forgenerating an electronic discharge and a supply of hydrogen (not shown),the molecules of which are bombarded by the electronic discharge to formprotons and molecular H2 ions. Voltages for maintaining the electronicdischarge within ion source I2 and for focusing the generated ions l maybe supplied by an alternating current generator I5 driven from pulley 5.The output of generator I5 may be rectified and employed to energize asuitable electronic high voltage supply circuit or circuits I6 whichperform the function of providing the correct focusing and electronicdischarge potentials for ion source I2. To apply high voltage to theions generated within source I2, connection to the terminal electrode 3may be made at a suitable point within supply I6, as indicated byconductor I1. It will be understood by those well skilled in the artthat accelerating tube I I and'exhaust tube I3 should be formed ofinsulating material having conductor rings spaced along the lengththereof for the purposes of grading the potential between terminalelectrode 3 and the face plate I8 of tank I to prevent spark over. Otherstructural considerations which must be observed in the construction ofelectrostatic generator apparatus as above described may be found in anarticle entitled Electrostatic Generator Operating Under HighPressure-Operational Experience And Accessory Apparatus, appearing inthe Physical Review, volume 53. page 642 (1938).

The ions supplied by ion source I2 are accelerated through the potentialdifference between terminal electrode 3 and face plate I8, which may bemaintained at ground potential as indicated at I9. within acceleratingtube II, which may be evacuated through a connection 20 to vacuum pumpI4. After this acceleration has been imparted to the ions, the protonsand molecular Hs ions may be caused to follow divergent paths by meansof a transverse magnetic field created by an electromagnet 2I energizedby direct current supply 22. As will be well understood by those skilledin the art, the protons have a smaller mass than the molecular H2 ions.and, hence, their trajectory will have a smaller radius within themagnetic ileld whereupon they may be directed through extension 23 ofaccelerator tube II along tube 24 to a target 25 for the purpose ofproducing a desired nuclear reaction. The molecular H2 ions, having alarger mass, will be influenced less than the protons by the magneticiield and, hence,

may be directed along extension 24 through an electrostatic analyzer 26to a pair of collector electrodes 21 and 28 suitably enclosed byenvelope 29. Since the molecular H2 ions are generally not desired forthe purpose of producing nuclear reactions as are the protons, they areseparated as above described by the magnetic field and employed for thepurpose of controlling the voltage on terminal electrode 3 in a mannerwhich will appear presently.

It will now be apparent that any variation in the voltage of terminalelectrode 3 will cause a similar variation in the energy of theaccelerated ion beam, inasmuch as the energy imparted to the ions willbe directly proportional to the accelerating voltage. As has beenmentioned heretofore, the energy spectrum of the protons must bemaintained within narrow limits in order that they may be advantageouslyutilized for nuclear reactions, such as the production of monoenergeticneutron beams. It is well known that electrostatic generators areinherently unstable, at least to the extentthat the terminal voltageproduced by such apparatus varies over a relatively wide range due tospark over, intermittent corona leakages, variations of chargecollection upon the terminal electrode, etc. Consequently. if suchapparatus is to be utilized advantageously in the aforementioned manner.means must be provided for regulating the terminal voltage withinrelatively narrow limits. According to the present invention, suchcontrol is provided by supplying a composite signal to a corona leakelectrode 30 whereby the charge leakage from and hence the terminalvoltage of, electrode 3 may be regulated as desired.

In order to derive a signal from the ion beam traversing acceleratingtube II responsive to changes in the voltage of terminal electrode 3,the molecular H2 ions are separated from the protons by means ofelectromagnet 2|, as hereinbefore mentioned, and then are focused at thedesired energy level by means of electrostatic analyzer 26, which servesas an energy selector, so that they fall in equal amount upon collectorelectrodes 21 and 28. Electrostatic analyzer 26 comprises a pair ofcurved deflecting plates 3I and 32 energized through conductors 33 and34 by a direct current power supply having the polarity indicated. Thepower supply 35 should have a manually operable output voltage controlso that the electrostatic field between plates 3l and 32 may be adjustedto the correct value for focusing the molecular H2 beam to fall equallyupon collector electrodes 21 and 28 at a desired voltage of terminalelectrode 3. Collector electrones 21 and 28 are connected respectivelyby conductors 36 and 31 .to a direct current amplifier 38. Amplifier 28should be of the type well known to those skilled in the art foramplifying a difference signal, l. e. have a zero output when thecurrents generated by molecular Hz ions falling upon collectorelectrodes 21 and 28 are equal in magnitude and an output differing fromzero when the ions are not evenly distributed between electrodes 21 and28. The output of amplifier 38 is fed through a mixer 39, which may beof the conventional resistance type, and a conductor l0 to acontrollable high voltage supply 4I. High voltage supply 4I energizescorona electrode 30, which may comprise a series of needles 42 supportedwithin tank I through an insulator 43 and a conductor 44.

As has been disclosed by R. N. Ashby and A. O. Hanson in an articleappearing in the Review of Scientific Instruments, volume 13, page 128(1942) corona electrode 30 may be employed as a variable corona leak tocontrol the charge upon and voltage of terminal electrode 3. The needles42 must, however, be maintained at a voltage below the voltage ofterminal electrode 3, but not excessively low. If the potential ofneedles 42 is too high, control of the charge upon terminal electrode 3is lost or spark over occurs between needles 42 and tank I; if thevoltage upon needles 42 is too low, spark over between needles 42 andelectrode 3 occurs. Therefore, ii the average voltage of high voltagesupply 4I is maintained within proper limits, control of the voltage ofterminal electrode 3 may be realized. Consequently, the signal derivedfrom the molecular Hz ion beam and fed into controllable high voltagesupply 4I may be employed as described to decrease the output of supply4I when the voltage of terminal electrode increases and to increase theoutput of voltage supply 4I when the voltage of terminal electrode 3decreases. With this negative feedback arrangement, a measure of controlof the terminal electrode voltage of the electrostatic generator may beobtained.

From experience, however, it has been found that, if the signal fromamplifier 38 alone is employed, a ripple voltage appears upon terminalelectrode 3. This is apparently due to the time constant of the transitof negative Oz ions from needles 42 to terminal electrode 3. Thistransit time will depend upon the spacing between needles 42 andterminal electrode 3 and upon the pressure within airtight tank I. For aparticular generator designed to operate at 3.5 m. e. v., this transittime has been found to be approximately 7 milliseconds at a pressure of150 pounds per sq. in. gauge in either nitrogen or air. Sufcient O2molecules are always present, due to gas occlusion in the walls of thetank I, etc., to serve as absorbers of all of the electrons produced atpoints 42 and, hence, the negative Oa ions are the actual chargecarriers rather than electrons themselves. The approximately 1millisecond lag in charge transference, together with the time constantsof circuit elements, results in a ripple frequency in the voltage ofterminal electrode 3 of about 10 to '70 cycles per second. In additionto the time required for charge transference. there is a delayencountered in the transit time of the ions along accelerating tube II,such time being of the order of l microsecond.

Since amplifier 38 must, of necessity, be a direct current amplifier inorder to amplify the current generated by molecular H2 ions strikingcollector plates 21 and 28, it is not practicable to compensate for thisripple frequency in the4 above-described feedback loop; because, if atthe ripple frequency the gain of this loop is increased sufficiently tocompensate for the ripple voltage, instability due to the long timedelay will result. According to the present invention, this difficultyis overcome by providing a second feedback loop responsive to voltagevariations of terminal electrode 3 whereby a composite signal may beobtained for controlling the potential of corona leak electrode 30. Thissecond feedback loop comprises a capacitive means 45, an alternatingcurrent amplifier 46, and a phase shifter 41. As illustrated in the moredetailed view of Fig. 2, capacitive means 45 comprises a capacitor plate48 supported with its inner surface essentially parallel to the innersurface of tank I by means of a stud 49, an insulator 50, a base 5I andan extension 5I of tank I. The whole assembly may be retained inairtight relationship against extension 5I by means of bolts 52, nuts 53and rubber gasket 54. Insulator 50 comprises a hollow outer sleeve 55and an inner hollow portion 56. through both of which extends stud 49 asshown. An airtight relationship is maintained here by means of rubbergaskets 51, clamp 58, washer 59 and nuts 6U. It will be apparent thatthe relative position of capacitor 48 with respect to the inner surfaceof tank I may be adjusted by means ofY stud 49 and nuts 60.

It will now be observed that the capacitance from terminal electrode 3to capacitor plate 48 in series with the capacitance from plate 48 totank I and the capacitance of a coaxial input lead 6I interconnectingstud 49 and the input to amplifier 46 will form a capacitance voltagedivider. From this divider, a proportion of the voltage changes ofterminal electrode 3 are fed back through amplifier 46, phase shifter 41and mixer 39 to the input of high voltage supply 4I, such a signal beinglimited only by the finite speed of light and not by the transit time ofcharge transference between capacitance plate 48 and terminal electrode3. Phase shifter 41, which may be of the common resistance-capacitancetype well known to those skilled in the art, is employed to advance thesignal derived from capacitance plate 48 to compensate for the transittime of the negative O2 ions between needles 42 and terminal electrode3, thus enabling the gain of alternating current amplifier 46 to beincreased considerably without the incurrence of instability in thecircuit. Mixer 39, which may be of the well known resistance type, isemployed to mix the signals from amplifier 38 and phase shifter 41,whereby a composite signal may be transmitted to high voltage supply 4I.By varying the amount of signal fed in from phase shifter 41 to mixer39, an optimum regulatory composite signal may be obtained to giveregulation within il kilovolt at 2.5 million volts.

The composite signal transmitted to high-volt age supply 4I in themanner hereinbefore described provides very good regulation for bothshort and long time intervals. However, it may be that after a sustainedperiod of operation the average voltage of high voltage supply 4I mayfall without the hereinbefore described range required by needles 42.Hence, an integrating circuit 62 may be connected between the output ofhigh voltage supply 4I and high voltage supply 9 for the purpose ofaltering the charge supplied to belt 2 thereby to compensate for longperiod variations of the voltage upon terminal electrode 3 which resultin the average voltage of needles 42 falling without the control range.

It will be understood by those skilled in the art that controllable highvoltage supplies 9 and 4I may be of a conventional type such as thosereferred to in an article appearing in the proceedings of the I. R. E.,volume 37, No. 2, page 199 (February 1949). Suitable circuits for directcurrent amplifier 38 and alternating current amplifier 46 may be foundin the voluminous literature describing such amplifiers.

In addition to the advantages of the invention mentioned hereinbefore,it will be noted that the signal derived from capacitance plate 48 doesnot depend upon the existence of an ion beam within accelerating tubeII; hence, this signal may be employed to regulate the potential uponterminal electrode 3, as above described, even though an ion beam is notbeing accelerated and the generator is employed to energize adissipative load in a manner well known to those skilled in the art.

While the invention has been described by reference to particularembodiments thereof, it will be understood that numerous changes may bemade by those skilled in the art without actually departing from theinvention. I, therefore, aim in the appended claims to cover all suchequivalent variations as fall within the true spirit and scope of theforegoing disclosure.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. The system of controlling the terminal voltage of an electrostaticgenerator having a charge collecting electrode, means for transportingelectrical charge to said electrode, and a corona leak electrode forvarying the charge leakage from said collecting electrode, said systemcomprising voltage supplying means having an output variable in responseto an input signal for supplying voltage to said corona leak electrode,means responsive to variations in the voltage of said collectingelectrode for deriving a signal substantially instantaneously responsiveto variations of said collecting electrode voltage, and a feedback loopinterconnecting the input of said variable output voltage supplyingmeans and said last mentioned means whereby the potential of said coronaleak electrode may be substantially instantaneously varied to controlthe charge leakage from and hence the voltage of said collectingelectrode.

2. The system of controlling the terminal voltage of an electrostaticgenerator having a terminal electrode, means for transporting electricalcharge to said electrode, and a corona leak electrode for varying thecharge leakage from said terminal electrode, said system comprisingvoltage supplying means having an output variable in response to aninput signal for supplying voltage to said corona leak electrode, meansresponsive to variations in the voltage of said terminal electrode forderiving a signal substantially instantaneously responsive to variationsof said terminal electrode voltage, phase shifting means for advancingthe phase of said signal derived from said terminal electrode voltage,and a feed-back loop interconnecting said phase shifting means and theinput of said variable output voltage supplying means whereby thepotential of said corona leak electrode may be substantiallyinstantaneously varied to control the charge leakage from and hence thevoltage of y voltage supplying means having an output variable inresponse to an input signal for supplying voltage to said corona leakelectrode, means for deriving a signal from said ion beam responsive tochanges in the voltage of said terminal electrode, capacitive meansresponsive to variations in the voltage of said terminal electrode forderiving a signal substantially instantaneously responsive to variationsof said terminal electrode voltage, means for mixing said signalsderived from said ion beam and said capacitive means to obtain acomposite signal, and a feedback loop interconnecting said mixing meansand the input of said variable output voltage supplying means wherebythe potential of said corona leak electrode may be varied in response tosaid composite signal to control the charge leakage from and hence thevoltage of said terminal electrode.

4. The system of controlling the terminal voltage of an electrostaticion accelerator having a terminal electrode, means for transportingelectrical charge to said electrode to build up a high terminal voltage,means for generating and accelerating ions in a beam to a high energywith said terminal voltage, and a corona leak electrode for varying thecharge leakage from said terminal electrode, said system comprisingvoltage supplying means having an output variable age to said coronaleak electrode, means for deriving a signal from said ion beamresponsive to changes in the voltage of said terminal electrode,capacitive means responsive to variations in the voltage of saidterminal electrode for deriving a signal substantially instantaneouslyresponsive to variations of said terminal electrode voltage, phaseshifting means in circuit with said capacitive means for advancing thephase of said signal derived from said capacitive means, means formixing said signals derived from said ion beam and said capacitive meansto obtain a composite signal, and a feedback loop interconnecting saidmixing means and the input of said variable output voltage supplyingmeans whereby the potential of said corona leak electrode may be variedin response to said composite signal to control the charge leakage fromand hence the voltage ci' said terminal electrode.

5. The system of controlling the terminal voltage of an electrostaticion accelerator having a terminal electrode, a moving charge carrier andvariable charge supplying means for spraying charge upon said carrierfor transportation to said electrode to build up a high terminalvoltage, means for generating and accelerating ions in a beam to a highenergy With said terminal voltage, and a corona leak electrode forvarying the charge leakage from said electrode, said system comprisingvoltage supplying means having an output variable in response to aninput signal for supplying voltage to said corona leak electrode, meansfor deriving a signal from said ion beam responsive to changes in thevoltage of said terminal electrode, capacitive means responsive tovariations of said terminal electrode for deriving a'signalsubstantially instantaneously responsive to variations of said terminalelectrode voltage, phase shifting means in circuit with said capacitivemeans for advancing the phase of said signal derived from saidcapacitive means, means for mixing said signals derived from said ionbeam and said capacitive means to obtain a composite signal, a feedbackloop interconnecting said mixing means and the input of said variableoutput voltage supplying means whereby the potential of said corona leakelectrode may be varied in response to said composite signal to controlthe charge leakage from and hence the voltage of said terminalelectrode, and means including an integrating circuit interconnectingsaid variable charge supplying means and said variable output voltagesupplying means for maintaining the average potential of said coronaleak electrode Within desired control limits.

6. Electrostatic generator apparatus comprising a terminal electrode,means for transporting electrical charge to said electrode to build up ahigh terminal voltage, an airtight conductive tank enclosing saidterminal electrode and said charge transporting means, a corona leakelectrode disposed Within said tank, variable voltage supplying meansconnected to said corona leak electrode for varying the charge leakagefrom said terminal electrode, a capacitance voltage divider including acapacitance plate disposed within said tank in opposed relation to saidterminal electrode for deriving a signal substantially instantaneouslyresponsive to variations in the voltage of said terminal electrode, andmeans for feeding said signal back to said variable voltage supplyingmeans whereby the potential of said corona leak electrode may besubstantially in response to an input signal for supplying volt- Ilinstantaneously varied to control the charge leakage from and hence thevoltage of said terminal electrode.

7. Electrostatic generator apparatus comprising a charge collectingelectrode, means for transporting electrical charge to said electrode, acorona leak electrode for varying the charge leakage from saidcollecting electrode, voltage supplying means having an output variablein response to an input signal for supplying voltage to said corona leakelectrode, means including a capacitor plate positioned within theelectric iield of said charge collecting electrode for deriving a signalsubstantially instantaneously responsive to variations of saidcollecting electrode voltage, and a feedback circuit interconnecting theinput of said variable output voltage supplying means and said lastmentioned means whereby the potential of said corona leak electrode maybe substantially instantaneously varied to control the charge leakagefrom and hence the voltage of said collecting electrode.

8. Electrostatic generator apparatus comprising a terminal electrode,means for transporting electrical charge to said electrode, a coronaleak electrode for varying the charge leakage from said terminalelectrode, voltage supplying means having an output variable in responseto an input signal for supplying voltage to said corona leak electrode,means including a capacitor having at least one plate located in theelectric field of said terminal electrode for deriving a signalsubstantially instaneously responsive to variations of said terminalelectrode voltage, phase shifting means for advancing the phase of saidsignal derived from said terminal electrode, and a feedback circuitinterconnecting said phase shifting means and the input of said variableoutput voltage supplying means whereby the potential of, said,I coronaleak electrode may be substantially instantaneously varied to controlthe charge leakage from and hence the voltage of said terminalelectrode.

9. Electrostatic ion accelerator apparatus comprising a terminalelectrode, means for transporting electrical charge to said electrode tobuild up a high terminal voltage, means for generating and acceleratingions in a beam to a high energy with said terminal voltage, a coronaleak electrode for varying the charge leakage from said terminalelectrode, voltage supplying means having an output variable in responseto an input signal for supplying voltage to said corona leak electrode,means for deriving a signal from said ion beam responsive to changes inthe voltage of said terminal electrode, means including a capacitorhaving at least one plate located within the electric field of saidterminal electrode for deriving a signal substantially instantaneouslyresponsive to variations of said terminal electrode voltage, means formixing said signals derived from said ion beam and said last mentionedmeans to obtain a composite signal, and a feedback circuitinterconnecting said mixing means and the input of said variable outputvoltage supplying means whereby the potential of said corona leakelectrode may be varied in response to said composite signal to controlthe charge leakage from and hence the voltage of said terminalelectrode.

10. Electrostatic ion accelerator apparatus comprising a terminalelectrode, means for transporting electrical charge to said electrode tobuild up a high terminal voltage. means for generating and acceleratingions in a beam to a high energy with said terminal voltage, a coronaleak electrode for varying the charge leakage from said terminalelectrode, voltage supplying means having an output variable in responseto an input signal for supplying voltage to said corona leak electrode,means for deriving a signal from said ion beam responsive to changes inthe voltage of said terminal electrode, means including a capacitorhaving at least one plate located within the electric eld of saidterminal electrode for deriving a signal substantially instantaneouslyresponsive to variations of said terminal electrode voltage, phaseshifting means in circuit with said means including said capacitor foradvancing the phase of said signal derived from said last mentionedmeans, means for mixing said signals derived from said ion beam and saidmeans including said capacitor to obtain a composite signal, and afeedback circuit interconnecting said mixing means and the input of saidvariable output voltage supplying means whereby the potential of saidcorona leak electrode may be varied in response to said composite signalto control the charge leakage from and hence the voltage of saidterminal electrode.

1l. Electrostatic ion accelerator apparatus comprising a terminalelectrode, a moving charge carrier and variable charge supplying meansfor spraying charge upon said carrier for transportation to saidelectrode to build up a high terminal voltage, means for generating andaccelerating ions in a beam to a high energy with said terminal voltage,a corona leak electrode for varying the charge leakage from saidelectrode, voltage supplying means having an output variable in responseto an input signal for supplying volt- ,age to said corona leakelectrode, means for deriving a signal from said ion beam responsive tochanges in the voltage of said terminal electrode, means including acapacitor having at least one plate located within the electric field ofsaid terminal electrode for deriving a signal substantiallyinstantaneously responsive to variations of said terminal electrodevoltage, phase shifting means in circuit with said means including saidcapacitor for advancing the phase of said signal derived from said lastmentioned means, means for mixing said signals derived from said ionbeam and said means including said capacitor to obtain a compositesignal, a feedback circuit interconnecting said mixing means and theinput of said variable output voltage supplying means whereby thepotential of said corona leak electrode may be varied in response tosaid composite signal to control the charge leakage from and hence thevoltage of said terminal electrode, and means including an integratingcircuit interconnecting said variable charge supplying means and saidvariable output voltage supplying means for maintaining the avaragepotential of said corona leak electrode within desired control limits.

HENRY W. BOUSMAN.

REFERENCES CITED The following references are of record in the ille ofthis patent:

Generating Voltmeter for Pressure-Insulated High Voltage Sources byTrump et al., pp. 54-56 of Review of Scientific Instruments, vol. 11,No. 2, February, 1940.

